This invention relates to automatic dishwashing tablets with improved chlorine stability. More specifically, it relates to the use of certain nonionic surfactants in machine dishwashing detergent tablets containing chlorinating agents.
Most machine dishwashing detergents contain a chlorine bleach component that provides sanitization and stain removal properties. Incorporation of nonionic surfactants in these detergent formulations is also highly desirable. The nonionic surfactants typically used are low foaming, are good defoamers of protein food soils, and provide wetting properties to reduce the appearance of spots and film on dishware, thus providing good sheeting action.
Chlorinating agents degrade conventional defoaming nonionic surfactants such as ethylene oxide/propylene oxide block copolymers and fatty alcohol oxyalkylates. As the chlorinating agent attacks the nonionic surfactant, the bleach is depleted and the surfactant is destroyed. Thus, desirable low-spotting filming and defoaming properties are lost along with the sanitizing and de-staining properties of the chlorinating agent.
Nonionic surfactants are commonly incorporated in automatic dishwashing detergent powders containing a chlorinating agent. In powdered detergents, the contact between the nonionic surfactant and the chlorine source is less intimate than in pressed tablets and stability can be maintained for the shelf life of the product.
In tablets, on the other hand, chlorine stability is significantly poorer than in powdered detergent formulations. One reason may be that the compression of the ingredients to form the tablet increases the contact between the nonionic surfactant and the chlorine source and degradation is accelerated.
Pressed automatic dishwasher detergent tablets, which have become more popular with the consumer over the past few years, present a special challenge for the formulator to produce a tablet with sufficient chlorine stability such that the sanitization and stain removing properties remain high throughout the shelf life of the product. Thus, from the time that the tablet is manufactured to the time of purchase and up until ultimate end use by the consumer, the shelf life may be six months or longer. An ideal formulation would retain most or all of its chlorine stability over that time period.
In order to predict the shelf life, it has been found convenient to determine the chlorine content in the tablet as a function of time during an accelerated aging test at elevated temperatures. As a rule of thumb, a tablet has sufficient chlorine stability if greater than 80 percent of the chlorine in the tablet remains active for eight weeks with the tablet stored at 120xc2x0 F.
Automatic dishwashing tablets containing nonionic surfactants and a chlorine bleach compound are disclosed in U.S. Pat. No. 6,025,315 to Gorlin, the disclosure of which is hereby incorporated by reference. According to Gorlin et al ""315, an automatic dishwashing composition in the form of a table comprises an alkali metal phosphate detergent builder salt, an alkali metal carbonate, a lubricant, a dialkali metal disilicate, a nonionic surfactant, and a wax-coated chlorine bleach compound.
Multiple layer detergent tablets are also known in the art as evidenced by U.S. Pat. No. 5,133,892 to Chun et al (the entire content of which is incorporated hereinto by reference. Multiple layer tablets are useful because incompatible materials can be incorporated into different layers, with an increase in shelf life.
According to the present invention, certain nonionic surfactants have been found to provide improved chlorine stability in machine dishwashing detergent tablet formulations (i.e., exhibit greater than 80 percent of the chlorine in the tablet remains active for eight weeks with the tablet stored at 120xc2x0 F.) as compared to the alcohol alkoxylate nonionic surfactants that are typically used in such an end-use application. The use of these nonionic surfactants allow such formulations to retain their cleaning and defoaming properties for longer periods of time as compared to formulations containing conventional nonionic polyether surfactants.
According to particularly preferred embodiments, there is provided according to the present invention a solid automatic dishwasher detergent tablet comprising a chlorine source to provide 0.5 to 5 percent available chlorine; and 1 to 10 percent by weight of one or more nonionic surfactants having the following structure
Rxe2x80x94(AO1)x(AO2)yxe2x80x94ORxe2x80x2
wherein:
R is a C6-C18 alkyl group,
AO1 represents propylene oxide or a mixture of propylene oxide and other alkylene oxides,
AO2 represents ethylene oxide or a mixture of ethylene oxide and other alkylene oxides,
Rxe2x80x2 is an alkyl, arylalkyl, or benzalkyl group with 1 to about 10 carbon atoms,
x is 2-20, y is 0-20, and the sum of x plus y is 5 or greater.
In another embodiment, a method for increasing the chlorine stability of the pressed automatic dishwashing detergent tablet is provided, wherein there is incorporated in the tablet a surfactant of the structure given above.
In accordance with the invention, the chlorine stability of the tablet is such that greater than 80 percent of the chlorine in the table remains active for eight weeks when the tablet is stored at 120xc2x0 F.
These and other aspects and advantages will become more apparent after careful consideration is given to the following detailed description of the preferred exemplary embodiments thereof.
The automatic dishwasher detergent tablets of the invention necessarily comprise a chlorine source. The chlorine source is most preferably present in an amount to provide from about 0.1 to about 5 percent available chlorine by weight, preferably from about 0.5 to 5 percent available chorine, and most preferably from about 0.5 to 3 percent available chlorine.
Conventional chlorine bleach compounds may be employed in the compositions of this invention, such as dichloroisocyanurate, dichlorodimethylhydantoin, or chlorinated trisodium phosphate. Other examples of chlorine sources include sodium hypochlorite, potassium hypochlorite, monobasic calcium hypochlorite, dibasic magnesium hypochlorite, potassium dichloroisocyanurate, sodium dichloroisocyanurate, trichlorocyanuric acid, 1,3-dichloro-5,5-dimethylhydantion, N-chlorosulfamide, Chloramine T, Dichloramine T, Chloramine B, and Dichloramine B.
Preferred chlorine sources are chlorinated isocyanurates including potassium dichloroisocyanurate, sodium dichloroisocyanurate, sodium dichloroisocyanurate dihydrate, trichlorocyanuric acid, and dichloroisocyanurate. A commercially available chlorine compound is ACL-59 from Monsanto, which is potassium dichloro-s-triazenetrione (i.e. potassium dichloroisocyanurate).
It is typical to formulate dishwasher tablets to a desired chlorine level. Thus, preferred formulations of the invention contain from about 0.5 to about 5 percent available chlorine. Thus, the actual amount of the chlorine compound(s) noted above should be present in the tablet at a level somewhat greater than the desired available chlorine level to ensure that the desired level of chlorine is provided. For example, the use of 3.3 percent ACL-59 provides about 2 percent available chlorine in a pressed dishwasher detergent tablet formulation of the present invention.
Chlorine stable surfactants are incorporated into the dishwasher detergent tablet compositions of the present invention. Chlorine stable surfactants of the invention are represented by the formula
Rxe2x80x94(AO1)x(AO2)yxe2x80x94ORxe2x80x2.
In the formula, R is a C6 to C18 alkyl group. For example, R can represent the residue of a fatty alcohol having 6 to 18 carbon atoms. Examples of such fatty alcohols include decyl alcohol, lauryl alcohol, and stearyl alcohol, as well as the commercially available blends of alcohols, such as C12 to C15 alcohols, or C10 to C13 alcohols. In a preferred embodiment, R will be in the range of approximately C12 to C15.
Also in the formula, AO1 represents an alkylene oxide or mixture of alkylene oxides which are polymerized onto the fatty alcohol to form a first block. The composition of AO1 will vary, but it is preferred that AO1 comprise propylene oxide. Thus, AO1 can be all propylene oxide, or it can comprise a heteric blend of propylene oxide and ethylene oxide. When AO1 is a heteric blend of propylene oxide and ethylene oxide, then (AO1)x represents a heteric block of propylene oxide and ethylene oxide. When AO1 is all propylene oxide, (AO1)x represents a propylene oxide block. By the term xe2x80x9cheteric blendxe2x80x9d, and the like is meant a blend of two or more oxides of a given ratio. By the term heteric block, is meant the block of mixed oxides resulting from the addition of a heteric blend to a molecule.
Similarly, AO2 represents an alkylene oxide or mixture of alkylene oxides with the proviso that the relative composition of AO2 is different from AO1. Preferably, AO2 comprises ethylene oxide. When AO2 is all ethylene oxide, then (AO2)y is an ethylene oxide block. When AO2 comprises ethylene oxide and another alkylene oxide, for example, propylene oxide, then (AO2)y is a heteric block of ethylene oxide and propylene oxide.
The end group Rxe2x80x2 can be an alkyl, arylalkyl, or benzalkyl group with 1 to 10 carbon atoms. A particularly preferred Rxe2x80x2 is methyl.
The subscript x is from about 2 to about 20, while y can be from 0 to about 20.
In a preferred embodiment, y is non-zero, and x and y are both less than 10. In this embodiment, the chlorine stable surfactant of the present invention can be described as a capped fatty alcohol-initiated diblock alkoxylate, wherein the first block is based on propylene oxide or propylene oxide and another alkylene oxide; the second block is based on ethylene oxide or a mixture of ethylene oxide and another alkylene oxide; and the capping group is an alkyl, arylalkyl, or benzalkyl group containing 1 to about 10 carbon atoms, with the preferred capping group being methyl.
A preferred surfactant where y is non-zero comprises one where x and y are both less than or equal to about 10, where the first block (AO1)x is a propylene oxide block, and the second block (AO2)y is an ethylene oxide block. In a more preferred embodiment, x is less than y, so that the propylene oxide block is smaller than the ethylene oxide block. In a particularly preferred embodiment, x is less than or equal to about 5, and y is greater than x.
In another embodiment, the subscript y in the above surfactant structure is zero and the subscript x is from about 2 to about 20, so that the surfactant of the invention contains a single alkylene oxide block. In this case, the surfactant can be referred to as a capped fatty alcohol alkoxylate, where AO1 represents either propylene oxide or a heteric blend of propylene oxide and another alkylene oxide such as ethylene oxide. In this case, it is preferred that AO1 comprise a heteric blend of propylene oxide and ethylene oxide, and more preferred that the number of propylene oxide units in the heteric blend be greater than the number of ethylene oxide units. As noted above, alkoxylates having from about 2 to about 20 alkylene oxide units are preferred, as indicated by the value of the subscript x being from about 2 to about 20. The subscript x preferably has a value of less than or equal to about 10.
The automatic dishwasher detergent tablets of the invention further comprise conventional dishwashing detergent additives. Such additives include, for example:
a) 10 to 50 percent by weight of detergent builder salts, such as sodium tripolyphosphate;
b) 10 to 50 percent by weight of a dialkali metal disilicate or alkali metal silicate or mixtures thereof, examples of which include sodium metasilicate pentahydrate;
c) 5 to 50 percent by weight of an alkali metal carbonate or alkali metal citrate, such as, for example, sodium carbonate or sodium citrate; and/or
d) other conventional additives such as sodium sulfate or 0.5 to 5 percent of a lubricant or optional pigments, dyes and fragrances.
The tablets of the present invention may also contain polymeric additives useful as solubilizing agents or dissolution aids.
The automatic dishwashing tablets of the current invention are prepared by incorporating the chlorine source and the chlorine stable nonionic surfactant described into a tablet using known methods. In single layer tablets, all of the ingredients are pressed together in a single layer. This can lead to stability problems if the ingredients are incompatible with one another. As noted above, the current invention solves the problem of incompatibility of the nonionic surfactant and chlorine bleach components in the situation where they are incorporated into the same layer.
Alternatively, the tablets can be multiple layer tablets. For example, the incompatible chlorine source and nonionic surfactant could be separated into different layers. The chlorine bleach component may optionally also be separated from enzymes which are deactivated by contact with bleach. Using multiple layers, the formulator can also make tablets with different acting layers, such as one layer which dissolves in cold water, and another which dissolves in warm water. The multiple layers containing different active ingredients can be fused directly to each other, or they can be separated by barrier layers.
The present invention therefore provides for tablets which contain at least one layer (i.e., the table can be either single layer or multiple layer) where a chlorine source and a nonionic surfactant are combined as an intimate homogenous mixture with one another, resulting in a tablet with good chlorine stability, especially as shown by its performance in an accelerated aging storage test at 120xc2x0 F.
The capped chlorine stable surfactants of the present invention can be prepared by alkylating or endcapping a surfactant starting material having the structure
Rxe2x80x94(AO1)x(AO2)yxe2x80x94OH
where R, AO1, AO2, x, and y are as defined above. The alkylation step can be accomplished for example by reacting dimethylsulfate with the surfactant starting material. The surfactant starting material can also be treated with an alkaline material such as potassium hydroxide and further reacted with an alkyl or aryl halide in a Williamson type ether synthesis. Alternatively, the starting material can be first de-protonated with an alkyl lithium reagent, and then alkylated with an alkyl or aryl halide. The chlorine stable surfactants in Examples 1 and 2 are synthesized by reaction with butyl lithium, and subsequent reaction with methyl iodide.
The present invention will be further understood by reference to the following non-limiting examples.